1. The Field of the Invention
The present invention relates to pressure transducers for medical use. More particularly, the present invention is directed to a novel pressure transducer for use in direct measurement of human blood pressure, and which includes a novel apparatus and method for calibrating the transducer.
2. The Prior Art
Perhaps the most frequently measured condition of a patient undergoing evaluation, diagnosis, or treatment is the patient's blood pressure. For example, blood pressure measurement and monitoring are regularly employed with patients suffering from shock or caridovascular ailments. Advantageously, by monitoring the blood pressure of these and other types of patients, medical personnel are better able to detect blood flow difficulties and other cardiovascular problems at an early stage. As a result, the use of blood pressure measurement and continuous monitoring may increase the likelihood that a patient can be successfully diagnosed and treated with needed emergency assistance.
A variety of methods are currently used for measuring and/or monitoring blood pressure. For example, medical personnel frequently use various indirect blood pressure measurement techniques, such as measuring a patient's blood pressure by using a pressure cuff and a stethoscope. Blood pressure measurements can also be made by using invasive techniques which permit direct measuring and monitoring of blood pressure. Notably, when diagnosing and treating critically ill patients, direct blood pressure measurements are greatly preferred over indirect measurements.
This preference for direct blood pressure measurement is due to several factors. First, the use of direct blood pressure measurement greatly increases the accuracy of the blood pressure reading. Typical indirect techniques may, for example, yield errors as high as ten percent, whereas direct blood pressure measurement techniques are generally accurate to within about one percent. In addition, direct monitoring techniques facilitate the continuous monitoring of a patient's blood pressure on a beat-to-beat basis. Direct blood pressure monitoring also enables the rapid detection of a change in cardiovascular activity, and this may be of significant importance in emergency situations. Further, direct blood pressure monitoring techniques can be readily used to measure and monitor a patient's blood pressure at a specific internal location, such as within the chambers of the heart. Because of these and other advantages, therefore, direct blood pressure measurement and monitoring has become a routine procedure for many critically ill patients.
One of the most widely used techniques for direct blood pressure measurement and monitoring is called catheterization. In using this technique, a needle is first inserted into a peripheral blood vessel. For example, if it is desired to monitor arterial blood pressure, the needle may be inserted into the radial artery. If, on the other hand, venous blood pressure is to be monitored, the needle may be inserted into the antecubital, radial, jugular, or subclavian veins.
Once the needle is properly inserted, a special catheter is threaded through the needle and into the blood vessel. The catheter is filled with a sterile solution, such as a sterile saline solution. In addition, the catheter may be formed so as to facilitate the further threading of the catheter along the blood vessel. Thus, the catheter may be threaded through the needle and along the blood vessel until the tip of the catheter, which is located inside the blood vessel, is positioned at the particular point within the body at which it is desired to make the blood pressure measurement. Then, with the catheter thus in place, the needle may be withdrawn.
Prior to positioning the proximal end of the indwelling catheter within a patient as described above, the distal end of the catheter is connected to pressure tubing that is connected to a pressure transducer. The catheter is generally also connected to a suitable continuous flush device or heparin drip to help prevent clotting around the tip of the catheter. The pressure transducer is also electrically connected to some type of monitor device near the patient's bedside. Typical monitor devices include cathode-ray tube display devices, digital display and/or recording devices, printers, and plotters.
In addition to the proper set-up of the measurement equipment in the above-described manner, it is also highly important to prime the catheter and tubing with the sterile solution so that any air bubbles within the catheter and tubing are removed such that a continuous fluid column is provided from the pressure transducer to the tip of the catheter. Then, when the catheter is positioned within the patient's blood vessel, as the patient's heart thereafter pumps blood, periodic pressure pulses are transmitted through the patient's blood vessle and along the fluid column in the catheter to the pressure transducer. The pressure transducer generates electrical signals representing the pressure pulses, and such signals are then amplified and displayed by the monitor device. Usually, the monitor device is used to display the patient's blood pressure as a function of time, this type of display being commonly referred to as the blood pressure waveform. The patient's blood pressure waveform can then be used by medical personnel to appropriately diagnose and treat the patient.
It will be readily appreciated that one of the most important components of the above-described blood pressure monitoring system is the pressure transducer. Significantly, the accuracy and the precision of the pressure transducer set an upper limit to the quality of the blood pressure data which can be obtained. Therefore, those skilled in the art of blood pressure monitoring have attempted to develop pressure transducers which have a high degree of reliability, sensitivity, and accuracy.
A typical pressure transducer for use in blood pressure monitoring systems comprises a thin diaphragm which is capable of being deflected by the pressure pulses which travel through the fluid column in the catheter and tubing. Some type of mechanism is also provided for measuring the deflection of the diaphragm, usually comprising suitable electronic circuitry which is configured so as to generate an electrical signal representing the pressure exerted on the diaphragm.
While a variety of electronic mechanisms have been used to measure the deflection of a diaphragm in pressure transducers, perhaps the most common measuring mechanism which is currently in use comprises a resistive strain gauge, such a mechanism being quite similar to strain gauges that are commonly used in industrial applications. Basically, a resistive strain gauge comprises a thin resistive wire which is connected to the pressure diaphragm such that the wire is stretched whenever the diaphragm is deflected. In accordance with well-known principles, such a stretching of the wire causes the electrical resistance of the wire to increase. Assuming, therefore, that a constant voltage is being applied across the wire, such an increase in the wire's resistance will result in a corresponding decrease in the electrical current through the wire in accordance with Ohm's law. Thus, by continuously measuring the current through the wire, it is possible to obtain an electrical signal which represents the amount by which the diaphragm is being deflected and which, therefore, also represents the pressure being exerted on the diaphragm.
In order to increase the sensitivity and accuracy of the pressure measurement, it is common to connect four such resistive wires to a single pressure diaphragm. Typically, the wires are also connected together in a conventional Wheatstone bridge configuration. Moreover, two of the wires are connected to the diaphragm so as to be stretched when the diaphragm is deflected, while the other two wires are compressed as the diaphragm is deflected. A more recent technology involves state of the art integrated circuitry. By special processing the above-mentioned diaphragm can be made out of silicon with resistive material (such as Boron) diffused into the silicon in the form of a Wheatstone bridge. This makes the transducer more rugged, less expensive, and a more stable device. Significantly, using this type of diaphragm/circuitry configuration, it is possible to obtain quite precise measurements of even small pressure pulses acting on the pressure diaphragm.
Before a pressure transducer such as that described above can be used, however, it must be balanced and calibrated with the monitor to ensure that the readings it produces are accurate.
A transducer is balanced in order to establish atmospheric pressure as the baseline or zero point from which the patient's pressure is read. A transducer is often used with a disposable dome that fits over the transducer diaphragm. The dome has two ports, one on the side and one vertical. The side port is connected to the indwelling patient catheter after it is primed with saline solution. The other port is generally used for balancing and calibration.
In order to balance the transducer, the vertical port is opened to the atmosphere and the other port is shut off from the patient. The transducer is then raised or lowered until the top of the vertical port is level with the position of the indwelling tip of the catheter. For each inch off the proper level, there will be an error in the pressure reading of about 2 millimeters of mercury. The monitor is then zeroed and the transducer port recapped.
After the transducer has been balanced, it is often necessary to calibrate the system to compensate for electrical inaccuracies in the transducer (often termed the "cal factor"). With prolonged use, transducers lose some sensitivity and become less accurate, thus making it necessary to calibrate the monitor to the transducer so that the monitor will be appropriately adjusted to compensate for such inaccuracy.
The most reliable method of calibrating a pressure monitor to a transducer involves the use of a common mercury sphygmomanometer from which the pressure cuff has been removed. The rubber tubing leading to the mercury column on the sphygmomanometer is secured to one arm of a Y-connector, and the bulb that is otherwise used to inflate the pressure cuff is connected to the other arm of the Y-connector. Finally, the foot of the Y-connector is connected to the vertical port of the disposable transducer dome.
After closing off a stopcock disposed between the transducer and the patient, the sphygmomanometer bulb is pumped until the mercury column indicates an appropriate calibration pressure, such as 100 mm Hg. Because of the Y-connector, the transducer diaphragm is also subjected to the same pressure. Most likely the monitor will indicate some pressure differing from that measured by the sphygmomanometer due to inaccuracy of the transducer. The monitor's sensitivity control should be adjusted until it gives a pressure reading identical to that of the mercury column, thereby calibrating the pressure monitor to the pressure sensed at the transducer so that it gives an accurate reading. This procedure may be repeated over a range of pressures to check linearity. The sphymomanometer is then removed, the vertical port of the disposable dome is capped, and the stopcock between the transducer and the patient is opened to permit the monitoring of the patient's blood pressure.
Although effective, the use of a mercury sphygmomanometer to calibrate a pressure monitoring system in the manner described above suffers from several disadvantages. For example, the need to interconnect the sphygmomanometer to the transducer apparatus followed by pumping of air into the system to increase the pressure gives rise to a substantial risk of introducing bacteria or other microorganisms into the transducer unless the entire calibration apparatus is sterilized. Such microorganisms may then be carried into the patient through the indwelling catheter. The possibility of infection carried through an indwelling catheter is of serious concern with respect to any patient; in a seriously ill patient such as is usually the case when using direct pressure monitoring, an infection introduced through an indwelling catheter may be life-threatening.
Another disadvantage of the use of a sphygmomanometer in calibrating a transducer in the manner described above is the introduction of a liquid-air interface at the position where the liquid column in the transducer apparatus comes in contact with the air column from the sphygmomanometer. If the level of liquid in the transducer is too low, or if the technician forgets to close off the indwelling catheter, air bubbles may be introduced into the patient line as the sphygmomanometer is pumped. Even a very small amount of air introduced into the patient's bloodstream could lead to formation of an air embolism which may seriously threaten the patient's health. Further, air bubbles that are introduced into the indwelling catheter, even if not introduced into the patient, are extremely disadvantageous because they are compliant and thus dampen the blood pressure pulses, thereby reducing the ability of the monitoring system to accurately record the patient's blood pressure.
Other techniques have also been devised in the prior art to ensure that the monitor's reading corresponds to the transducer's sensitivity. Such techniques include electronic calibration procedures wherein by depressing a shunt switch on the monitor a predetermined resistance which is provided in the circuitry of the monitor or cable/connector may be shunted across the transducer output. The disadvantage of this particular technique is that it does not test the accuracy of the transducer diaphragm itself. See, Bruner, John M. R., Handbook of Blood Pressure Monitoring, pp. 115-117 (1978). Thus, when using this calibration technique one must still periodically use a mercury sphygmomanometer to apply a known pressure to the transducer diaphragm so that the monitor can then be calibrated to that pressure. Accordingly, the above-described technique does not remove the difficulties inherent in maintaining sterility and in avoiding introduction of air into the patient line.
Still another approach that has been used in the prior art is a volumetric technique such as provided with a device having a known volume which can be coupled to the pressure monitoring system. An example of this approach is exemplified by the Bruton Industries B200 dynamic blood pressure calibrator. The device works well for purposes of placing a known pressure on the transducer in order to accurately determine the necessary calibration adjustment. However, various problems remain, such as maintaining sterility as well as the fact that the volumetric device must be adjusted for different altitudes. Also, typically volumetric calibration devices are relatively expensive and somewhat complicated in their construction and use.
Another relatively recent innovation which has been made in the state of the art of direct blood pressure monitoring systems in the use of disposable pressure transducers. Previously, pressure transducers which have been used in the state of the art were quite expensive and were used many times. In order to prevent contamination when using a transducer on a new patient, disposable domes were provided for the transducer so that only the plastic dome which is screwed onto the top of the transducer is required to be discarded before reusing the transducer with another patient. However, as mentioned above, after prolonged use the sensitivity and stability of the transducer may be adversely affected and it therefore may become necessary to more frequently calibrate the transducer.
As noted above, recent advances in electronic technology have made it possible to develop small, relatively inexpensive silicon chip transducers which are completely disposable after a single use. Nevertheless, even though the cost of such disposable transducers has been greatly reduced, typically the cost is still high enough that it would be advantageous to be able to reuse such transducers at least to a limited extent, thereby providing the potential for further cost savings. To date, it has not been possible to reuse such silicon chip transducers since they are interfaced directly to the fluid-filled indwelling catheter and therefore are contaminated once they have been used.
In view of the present state of the art, it will be appreciated that it would be a significant advance in the field of direct blood pressure monitoring if methods and apparatus could be provided for calibrating pressure transducers in a manner that would not expose the patient to the risk of infection, and in a manner that would prevent the risk of introducing air bubbles into the system. It would be a further significant advantage if a pressure transducer which is small and economical enough to be disposable could be adapted for reuse to a limited extent so as to further reduce the costs involved. Such methods and apparatus are disclosed and claimed herein.